Synthesis of reversibly cleavable hetero-bifunctional crosslinking reagents of which two dissimilar functional groups can be activated independently. To prepare binding proteins (lectins, toxins, hormones, and antibodies) which are reacted only with one of the two functional groups in the crosslinking reagents. To crosslink the binding proteins prepared as such to their receptors by activating the other functional group when the binding proteins are associated with the receptors, and to identify the binding protein-receptor complexes. To examine the surface receptors of normal and transformed cells in culture.